Since the RIS is usually mounted at some tall building, it is expected that there are only limited scatters around the RIS. This suggests the adoption of a sparse angular channel model [1]}, which has been validated by real-world measurements and widely adopted in RIS related research [2]}. In specific,
the RIS-BS channel, i.e., \({\bf U}\in \mathbb {C}^{M\times N}\)  can be expressed
as
\(\mathbf {U}=\sqrt{\bar{\mu }}\sum _{i=1}^{\bar{I}}\bar{\xi }_i\mathbf {a}_\mathrm {B}(\sigma _i)\mathbf {a}_\mathrm {R}(\bar{\phi }_i,\bar{\psi }_i)^H,\) 
